1. Field of the Invention
The present invention relates to a mask for screen printing (screen mask) including a positive pattern section and a negative pattern section for transferring a printing ink material to a substrate via mesh openings disposed on the positive pattern section. The present invention also relates to a method for producing such a screen mask, and a circuit board including a wiring pattern having at least a capacitor element, the wiring pattern being formed by screen printing.
2. Description of the Related Art
In general, the screen printing resides in a printing method based on the use of a screen mask comprising a mesh stretched over a frame, the screen mask including mesh openings of a negative pattern section, the mesh openings being closed by resin or the like (for example, a photosensitive emulsion film). In this method, a squeegee is allowed to slide on the upper surface of the screen mask so that an ink material is extruded toward the substrate through mesh openings of a positive pattern section of the screen mask to perform printing.
The screen printing makes it possible to perform printing on a variety of printing materials such as paper, cloth, plastic, glass, and metal, because the screen mask is soft and flexible, and the printing pressure is small. The screen printing also makes it possible to form a thick pattern composed of an ink material. Therefore, the screen printing is also applied to produce electronic parts such as thick film IC (hybrid IC), printed circuit boards, resistors, and capacitors.
Usually, the screen mask is manufactured such that a photosensitive emulsion film applied on a mesh is subjected to pattern formation by means of the photolithography technique. Alternatively, the screen mask is manufactured such that a metal film is subjected to selective etching to form a mesh pattern by means of the photolithography technique.
For example, the portion, at which the ink material is formed on the substrate through the mesh, forms a desired pattern. The portion corresponding to the photosensitive emulsion film formed on the mesh, or the portion corresponding to the non-mesh portion of the metal film forms a gap between the pattern.
In such a process, the mesh opening of the screen mask is about 100 xcexcm even in the case of the fine type. For example, an emulsion film, which has a width of not more than 40 xcexcm, is insufficiently supported by the mesh. Therefore, it is feared that such an emulsion film formed on the mesh is incapable of enduring the force applied by the sliding movement of the squeegee, and it is disengaged from the mesh.
The screen mask based on the metal film is originally insufficient in strength of the metal film. When the width of the non-mesh portion is not more than 80 xcexcm, such a screen mask is incapable of enduring the force applied by the sliding movement of the squeegee.
In other words, the conventional techniques involve the following problem. That is, even when it is intended to form a gap of not more than 40 xcexcm between the pattern by means of the one time of screen printing, it is difficult to form such a gap, because of the shortage of the holding force of the emulsion film and the strength of the metal film of the screen mask.
A method is conceived, in which the gap between the pattern is made to be not more than 40 xcexcm by performing the screen printing several times in a divided manner. However, in such a method, a large dispersion of not less than 10% arises in the difference between the film thickness of a pattern formed by an odd-numbered screen printing process and the film thickness of a pattern formed by an even-numbered screen printing process. For example, when the patterns are wiring patterns, any dispersion arises in the electric characteristic between these wiring patterns. Consequently, a new problem possibly arises in that it is impossible to obtain a desired device characteristic.
The present invention has been made taking such problems into consideration, an object of which is to provide a screen mask which makes it possible to obtain a gap between the pattern formed by one screen printing, the gap being not more than 40 xcexcm, and which makes it possible to form a fine pattern by using inexpensive screen printing.
Another object of the present invention is to provide a method for producing a screen mask with ease, in which the screen mask makes it possible to obtain a gap between the pattern formed by one screen printing, the gap being not more than 40 xcexcm.
Still another object of the present invention is to provide a circuit board in which a gap between the pattern formed by screen printing is not more than 40 xcexcm.
The present invention lies in a screen mask for screen printing including a positive pattern section and a negative pattern section with a mask material formed on the negative pattern section, for transferring a printing ink material to a substrate via openings of a mesh disposed at the positive pattern section, wherein the negative pattern section of the mesh selectively has a mesh opening ratio which is smaller than an opening ratio of the positive pattern section.
When the mesh opening ratio of the negative pattern section is decreased, then the width of each mesh of the negative pattern section is widened, and the contact area between the mask material and the mesh is increased. Accordingly, for example, it is possible to sufficiently hold a mask material which has a width of not more than 40 xcexcm.
As a result, the mask material, which is formed on the mesh, is sufficiently endurable to the force applied by the sliding movement of a squeegee during the screen printing. Thus, the mask material is not disengaged from the mesh. This results in the high reliability and the narrow width of the pattern gap formed by the screen printing. The width of the gap, which is formed on the substrate by the negative pattern section, can be made to be not more than 40 xcexcm. In other words, it is possible to form a fine pattern by using the inexpensive screen printing. Thus, it is possible to greatly reduce the production cost for forming the pattern.
In this arrangement, it is also preferable that a plating layer is formed on the mesh of the negative pattern section so that the mesh opening ratio of the negative pattern section is decreased. It is preferable that the plating layer has a thickness of 1 to 20 xcexcm, for example, in view of the plating treatment time and the holding force of the emulsion film.
In another aspect, the present invention lies in a screen mask for screen printing including a positive pattern section and a negative pattern section, for transferring a printing ink material to a substrate via openings of a mesh disposed at the positive pattern section, wherein the negative pattern section has a mesh opening ratio of zero.
That is, the screen mask has a form in which the negative pattern section is completely closed. Therefore, it is unnecessary to form any mask material on the negative pattern section, and it is possible to simplify the production steps. Further, it is unnecessary to consider, for example, the disengagement of the mask material. Therefore, it is possible to achieve the high reliability and the narrow width of the pattern gap obtained by the screen printing. The width of the gap, which is formed on the substrate by the negative pattern section, can be not more than 40 xcexcm.
In this arrangement, it is also preferable that the mesh opening ratio of the negative pattern section is made to be zero by forming a plating layer on the mesh of the negative pattern section.
In still another aspect, the present invention lies in a method for producing a screen mask for screen printing including a positive pattern section and a negative pattern section with a mask material formed on the negative pattern section, for transferring a printing ink material to a substrate via openings of a mesh disposed at the positive pattern section, wherein a plating treatment is selectively applied beforehand to the mesh of the negative pattern section so that the negative pattern section has a mesh opening ratio which is smaller than an opening ratio of the positive pattern section.
Accordingly, the width of each mesh of the negative pattern section is widened, and the contact area between the mask material and the mesh is increased. Therefore, for example, it is possible to sufficiently hold a mask material having a width of not more than 40 xcexcm.
As a result, it is possible to consequently obtain the high reliability and the narrow width of the pattern gap formed by the screen printing. The width of the gap, which is formed on the substrate by the negative pattern section, can be made to be not more than 40 xcexcm.
In the method described above, it is also preferable that at least a surface of both surfaces of a screen, on which a squeegee makes sliding movement, is polished after the plating treatment to give a flatness. Alternatively, it is also preferable that a plating mask material is formed before the plating treatment on a surface of both surfaces of a screen, on which a squeegee makes sliding movement so that the plating layer is not formed on the surface.
It is preferable that a plating layer is composed of a material which has a hardness lower than that of a screen so that the polishing treatment is easily performed.
In still another aspect, the present invention lies in a circuit board comprising a pattern formed by screen printing, the pattern including at least one of a passive device such as a capacitor element and an active device such as an electromechanical conversion element, wherein a gap between the pattern is not more than 40 xcexcm.
In the present invention, it is also preferable that the pattern is formed by one time of screen printing.
Conventionally, it is impossible to obtain a fine gap screen mask. Therefore, when a gap of not more than 40 xcexcm is formed, it is indispensable to perform a plurality of times of screen printing. However, it is inevitable that a printing pattern obtained in the second time is not formed under the same printing condition as that for a printing pattern obtained in the first time. For this reason, it is difficult to obtain a uniform film thickness.
On the contrary, in the present invention, the entire pattern can be formed by one time of screen printing. Therefore, it is possible to obtain a uniform film thickness. In this aspect, if it is assumed that a plurality of patterns are formed in an aligned manner, the difference between an average thickness of a pattern corresponding to a pattern assumed to be formed by an odd-numbered operation and an average thickness of a pattern corresponding to a pattern assumed to be formed by an even-numbered operation is not more than 5% of an overall average thickness.
It is also preferable that the screen printing is performed by using a screen mask including a positive pattern section and a negative pattern section with a mask material formed on the negative pattern section, for transferring a printing ink material to a substrate via openings of a mesh disposed at the positive pattern section, wherein the negative pattern section of the mesh selectively has a mesh opening ratio which is smaller than an opening ratio of the positive pattern section.
It is also preferable that a plating layer is formed on the mesh of the negative pattern section of the screen mask. In this arrangement, it is preferable that the plating layer has a thickness of 1 to 20 xcexcm.